This invention relates to a switching power supply, and in particular to a switching power supply suitable for driving a magnetron used in a microwave oven, etc.
A high voltage (e.g. a high voltage of an order of magnitude of kV) and a high electric power are necessary for driving a magnetron, whose current-voltage characteristics have constant voltage characteristics and vary depending on the temperature of the magnetron. FIG. 1 shows the relation between the anode current of the magnetron and the anode-cathode voltage with a parameter of the temperature of the magnetron by way of example. Further, when the intensity of the current increases over a critical value, it gives rise to an abnormal oscillation.
Thus, it is fairly difficult to drive stably a magnetron. For this reason, heretofore mostly used are power supplies of the type in which the AC voltage of the commercial frequency is stepped up by means of a transformer.
One of such prior art power supplies for driving a magnetron used in microwave ovens, etc. has been so constructed that commercial electric power is rectified by means of a half-wave voltage doubler circuit after having been stepped-up by using a transformer and supplied to the magnetron and the control of its output power has been effected by controlling the conduction phase of a bidirectionally controllable switching element connected in series with the primary winding of the transformer, as described e.g. in Denshi Gijutsu, Vol. 20, No. 3 (1978), p. 34-p. 45. On the other hand, recently small and lightweight switching power supplies have become available. However, in the case where they are used in a power supply for driving a magnetron in a microwave oven, etc., following problems are encountered. That is, when a magnetron is driven without load or with a light load, a part of emitted microwave is returned to the magnetron by reflection and heats itself, which raises the temperature of the magnetron and at the worst case gives rise to a risk to destroy the microwave output portion made of glass or ceramic. In the prior art power supply, in which the commercial frequency voltage is stepped-up by means of a transformer, since its output is automatically reduced for a light load, overheat of the magnetron due to the reflection is relatively small. On the contrary, in the case where the switching power supply is used, since an output previously set is emitted also for a light load, the overheat of the magnetron due to the reflection gives rise to a more serious problem.
In addition, the current-voltage characteristics of a magnetron vary, depending on the temperature, as indicated in FIG. 1. In this way, in the case where the temperature is increased, its operating voltage decreases and the current intensity increases for the same applied voltage. At the same time abnormal oscillation (moding) is caused when the current intensity exceeds a certain value, e.g. about 1 A in the case of a power supply for a microwave oven, etc. Consequently, it is necessary to regulate the input of the magnetron in dependence on the temperature.
In the prior art power supply disclosed in the above-mentioned literature, although the magnetron has constant voltage characteristics and also has an upper limit in the tolerable instantaneous electric power of its input, since the voltage applied to the magnetron has a large pulsation, the control margin of the output electric power is narrow and therefore there is a problem that it is necessary to turn off the power supply circuit by using a thermostat, etc. in order to stop the drive of the microwave oven etc.
In addition, in the prior art power supply described in the above-mentioned literature, since the transformer in the power supply of the magnetron in the microwave oven is operated on the commercial frequency source, this transformer is large and heavy and further it should be designed separately, depending on whether the used commercial frequency is 50 Hz or 60 Hz. The magnetron, which is the load, has the constant voltage characteristics, and when the voltage applied between the anode and the cathode exceeds the cut-off voltage, anode current begins to flow and increases linearly with increase in the applied voltage to generate a microwave output. However, as described earlier when the anode current reaches the critical value, the abnormal oscillation is caused and almost no microwave output is obtained. This abnormal oscillation greatly shortens the life of the magnetron. In addition, the cut-off voltage of the magnetron is e.g. approximately about 4 kV and this value varies, depending on the temperature of the magnetron. In a normal operating state of the magnetron it varies by several 100 V. However, in the prior art power supply, since no consideration is given to the change in the power supply output voltage and to the change in the cut-off voltage of the magnetron, there are problems that at the rise of the voltage of the power supply or at the lowering of the cut-off voltage the anode current exceeds the critical value and causes abnormal oscillation or that at the lowering of the voltage of the power supply or at the rise of the cut-off voltage the voltage applied to the magnetron becomes less than the cut-off voltage and almost no microwave output is obtained. Further, in order to resolve these problems, it is conceivable to determine the turn ratio of the transformer such that the voltage applied to the magnetron is higher than the expected highest cut-off voltage even if the power supply voltage decreases to its minimum value and to detect the anode current when the voltage of the power supply rises so as to turn off the bidirectionally controllable switch described above before the anode current reaches the critical value, to thereby prevent the abnormal oscillation. However, this scheme would cause problems such as necessities of a means for detecting the anode current and a complicated control circuit for the bidirectionally controllable switch.
As a prior art switching power supply for microwave ovens, there is known one disclosed in JP-A-No. 58-4121. In this switching power supply, the input frequency applied to the high voltage transformer is varied by using a frequency converter so that the oscillation output of the magnetron is varied. However, in this switching power supply it is not taken into account that the current-voltage characteristics of the magnetron vary, depending on the temperature, as described above. Consequently it doesn't permit to drive the magnetron under the optimum condition.
Further, in this switching power supply no consideration is given to the switching loss of a switching element used in the frequency converter.
In the case of resonance type converters, the method to turn-on the switching element at the point where the voltage applied thereto is lowest, in order to reduce the switching loss, is generally utilized. However, in these prior art resonance type converters, the resonance voltage is generated either by disposing a new resonance circuit on the input side of the transformer or by utilizing the circuit behavior on the output side of the transformer. Consequently, the operating margin is narrow and it cannot be used in the case, for example, where the input power source has an unsmoothed voltage waveform. On the other hand, for the switching power supply for a load such as a microwave oven which requires a high electric power, a large input current is necessary. Therefore, in order to satisfactorily smooth the input voltage, a capacitor having a large capacity is needed, which makes the power supply impractical.
It is hitherto not known to generate a resonance voltage utilizing only an exciting inductance and a capacitor for absorbing surge voltages, as proposed in this application.
In addition to the above-mentioned JP-A-No. 58-4121, a prior art switching power supply for the magnetron is disclosed in Japanese Utility Model Application Publication No. 55-33593.